As used herein, the term “Evaporative Light Scattering Detector” refers to an instrument comprising a nebulizer, a desolvation region, a light-scattering region, source of photons and a photo-detector for measuring the light scattered. ELSDs are capable of producing signals from a greater range of analytes than most other detectors, providing that the analytes are relatively involatile in comparison with the mobile phase in which the analytes are dissolved.
The term “sample” is used in the broadest sense to indicate something that one wishes to evaluate. Samples can originate with industrial materials, chemical synthesis, or may have originated from biological sources. The term “critical” refers to gases that are at critical temperature and pressure. The term supercritical refers to gases above such critical temperature or pressure. Gases will exhibit the solvation properties of a liquid at temperatures and pressure near the critical temperature and pressure. Supercritical fluids are used with cosolvents, or entrainers which modify, enhance or facilitate selection of a compound. This paper will refer to all such critical, near critical and supercritical fluids, and cosolvents and entrainers, as supercritical fluids unless the context of the text requires otherwise.
Chromatography is the science of separations based on differences in affinity that different compositions have to a stationary phase. High performance liquid chromatography (HPLC) is performed in columns or cartridges. Solutions in which samples are dissolved are pumped through the columns or cartridges. The columns and cartridges conduits have an inert stationary phase. The components of the sample separate as they move through the stationary phase due to the different affinity each compound has to the stationary phase. It desirable to detect the separated components with a detector. ELSDs are one form of detector that is used in chromatography.
This application will use the term HPLC as referring to separations at pressures up to approximately 3,000 pounds per square inch (psi). At higher pressures, it is possible to perform sharper better defined separations with greater speed. Higher pressures, referring now to the extreme high pressure range, approximately 4,000 psi to 15,000 psi, are now used. This paper will not distinguish between the high pressure range and the extreme pressure range and will use the term HPLC in reference to both unless the context requires otherwise.
The nebuliser of an ELSD is typically a pneumatic nebuliser. The nebuliser, when the ELSD is used with a chromatographic instrument, is disposed to receive the eluant from separation media and generate from it an aerosol that extends into the desolvation region. The desolvation region may comprise a heated drift tube wherein solvent is evaporated from the aerosol leaving a stream of particles. The particles then pass into the light scattering region where they pass through a light beam, typically disposed perpendicularly to the direction of travel of the particles, where they cause light to be scattered from the beam. A light detector is positioned to receive some of the scattered light and produces a signal indicative of the presence of the particles.
Unfortunately, prior types of ELSD sometimes lack the sensitivity of other types of detectors, and are often non-linear in their response to different quantities of a particular analyte, making quantitative use difficult.